The following disclosure is based on German Patent Application No. 100 24 986.8, filed on May 19, 2000, which is incorporated into this application by reference.
The present invention relates to an electronic weighing sensor having a digital signal processing unit, which includes at least one filter with a low-pass characteristic. By means of the filter, the direct-current component of the output signal of the weighing sensor is determined and the weighing result is derived therefrom.
Weighing sensors of this type are generally known in the art. The low-pass filter is used to suppress the alternating-current components, which are superimposed on the output signal of the weighing sensor when the installation site of the weighing sensor is subject to shocks or vibrations. Generally, despite this measure, the measuring results of weighing sensors and scales that are based on weighing sensors are clearly more difficult to reproduce if the installation site is subject to shocks or vibrations than if the installation site is steady.
To improve the performance of weighing sensors and scales at unsteady installation sites, it is known, for instance from U.S. Pat. No. 5,789,713, to provide a second, weighing sensor having a constant load whose output signal is used to derive a correction signal for the actual (measuring) weighing sensor. The mechanical and electronic complexity caused by this second weighing sensor is considerable, however.
Similarly, German laid-open document DE 40 01 614 A1 teaches the provision of at least one acceleration sensor instead of a second, constantly-loaded weighing sensor, which supplies a correction signal for influencing the measuring result. In both cases, however, the correction signal must be subtracted from the signal of the (measuring) weighing sensor in such manner that the phases of both signals are properly taken into account so that the interference can be corrected. This in-phase subtraction, however, can be achieved within only a limited frequency band. At the upper end of this frequency band, the phase shifts are more and more increased and differ between the measuring path and the correction path. This results, in the worst case, in an additionxe2x80x94and thus an amplificationxe2x80x94of the signals rather than a subtractionxe2x80x94and thus a cancellation. This is particularly true for acceleration sensors that are employed as correction generators, since the mechanically very differently structured system of the acceleration sensor has eigenfrequencies that are very different from those of the weighing sensor.
It is one object of the present invention to improve the performance of a weighing sensor in case of shocks and vibrations in the installation site without requiring a second weighing sensor or an acceleration sensor for correction purposes. Therein, problems due to phase shifts at the edge of the frequency range are to be avoided.
According to one formulation of the invention, this and other objects are achieved by providing a method for deriving a weighing result, in which a direct-current component of an output signal of an electronic weighing sensor is determined by means of a low-pass filter that is arranged in a digital processing unit. In addition, a further signal is determined by the digital processing unit that is dependent on an amplitude of vibrations of the electronic weighing sensor. The direct-current component is changed by electronic components in accordance with a magnitude of the further signal that is dependent on the amplitude of the vibrations.
This approach is based on the finding by the inventors that the poor reproducibility of weighing sensors at unsteady installation sites is not only caused by inadequate suppression of the alternating-current component in the output signal of the weighing sensor. Rather, a significant contribution to the poor reproducibility is due to the fact that the direct-current component in the output signal of the weighing sensor is influenced as a function of the amplitude and the frequency range of the alternating-current component. Thus, stronger suppression of the alternating-current component in the output signal of the weighing sensor alone does not sufficiently improve the performance of a weighing sensor that is subject to vibrations. In addition to that, a correction of the direct-current component must be carried out, as proposed by the present invention.
The causes for this influence on the direct-current component may be illustrated by three examples:
In the case of weighing sensors that have a non-linear characteristicxe2x80x94as shown in FIG. 1 in exaggerated formxe2x80x94and that are installed at a steady installation site, the load m1 is associated with the output signal corresponding to point A. In an unsteady site, however, the output signal fluctuates between the extreme points B and C along the non-linear characteristic B-A-C. Depending on the amplitude spectrum of the vibrations, the direct-current component of this output signal is located somewhere between points A and D. As a result, even if the alternating-current component in the output signal is completely suppressed, the direct-current component changes due to the non-linearity of the characteristic curve. This change is suppressed by the electronic components according to the present invention.
In the case of weighing sensors that operate based on the principle of electromagnetic force compensation, as illustrated schematically in FIG. 2, a coil 2, through which a current flows, is located in the magnetic field of a permanent magnet 1. The current flowing through the coil 2 is regulated by a position indicator 3 and by a downstream regulation amplifier 4 in such a manner that the electromagnetically generated force is precisely equal to force F to be measured. The magnitude of this current is measured at a measurement resistor 5 and is supplied to an output 6 as an output signal of the weighing sensor. As is well known, the quantitative relation between the magnetic field B, the current I and the generated force F is:
F≈Bxc2x7Ixe2x80x83xe2x80x83(1) 
To achieve optimal efficiency, the coil 2 is positioned in such a way that it is located at the point of the maximum magnetic field of the permanent magnet 1. If the coil 2 is caused to oscillate due to vibrations in the base, the coil 2 moves sometimes outside the magnetic field maximum and into a region with a lower magnetic field. On temporal average, the magnetic field B in equation (1) is thus lower than in a steady installation site where coil 2 is always located within the magnetic field maximum. Consequently, according to equation (1), a greater average current I is required to generate the same force F. Thus, in this example too, the direct-current component changes when there are shocks/vibrations so that, in addition to suppressing the alternating-current component, the direct-current component must also be corrected in order to obtain a stable and exact result.
If the characteristic curve of the position indicator 3 in the above-described weighing sensor according to FIG. 2 is asymmetrically non-linear, the average transient position is shifted when there are vibrations, as illustrated above by means of FIG. 1. On the one hand, this change in the average transient position causes a change in the effective magnetic field at the position of the coil, as shown in the second example. On the other hand, a slight deflection in the parallel guidance 7 of the scale tray 8 and coil 2 is caused by this change. If the parallel guidance 7 is realized by spring-type hinges, this causes a vertical spring force, which changes the direct-current component in the weighing signal.
Other causes, which are not further described here, may also distort the direct-current component in the weighing signal, e.g., non-linear amplifiers or non-linear transmission levers. Of course, all the described effects are small and, therefore, have consequences at high resolutions of the weighing sensor only.
Advantageous embodiments are set forth in the dependent claims.
The use of a high-pass filter for determining the vibration-dependent alternating-current components in the output signal of a weighing sensor and the control of a display element or a print lock (in case the alternating-current component exceeds a predefined limit value) is generally known in the art of scale technology. Such structures and methods are described, for instance, in German Patent DE 23 23 200. In the prior art, however, no influence is effected on the direct-current component of the output signal of the weighing sensor.